A microstrip antenna is typically comprised of a conductive plate, also known as a patch or a radiating element, that is separated from a ground plane by a dielectric material. The microstrip antenna is fed by applying a voltage difference between a point on the radiating element and a point on the ground conductor. Feed methods include direct feed such as probes or transmission lines and indirect feed such as capacitive coupling.
Microstrip antennas have a low profile, are light weight, are easy to fabricate and therefore, are relatively low cost. These advantages have encouraged the use of microstrip antennas in a wide variety of applications. In the automotive industry in particular, microstrip antennas are used on vehicles for receiving signals transmitted by Global Positioning System (GPS) satellites. Another automotive application includes using a microstrip antenna for a Satellite Digital Audio Radio System (SDARS) receiving antenna. While each of these applications can utilize a microstrip antenna, they each operate at different frequencies and require different polarizations and in the prior art would require separate antennas. As more and more applications are provided on a vehicle that require antennas to be integrated in the vehicle, dual-band and combination antennas provide a viable solution.
Most dual-band microstrip antennas known in the art utilize a stacking technique to obtain dual-band operation. Radiating elements are stacked on top of each other. While this conserves space in a lateral direction, it adds height which detracts from the advantage of the low-profile microstrip antenna. Further, the stacked patches are also subject to decreased performance. The performance of the lowest radiating element is degraded because it is blocked by the radiating element stacked above it. Therefore, the gain and beam width of the antenna may be compromised. An alternative to stacking is a co-planar microstrip antenna. However, interference is a concern with co-planar microstrip antennas. Most co-planar microstrip antennas incorporate slots for obtaining dual-band operation, yet are limited to linear polarization, and have limited bandwidth and gain characteristics. In order to avoid interference problems, co-planar microstrip antennas typically utilize multiple feed points in the feed network.
There is a need for a single microstrip antenna that is capable of operating in more than one frequency band, with more than one possible polarization and without sacrificing the advantages associated with microstrip antenna technology.